The transporting of heat transfer fluid, in particular refrigerant fluid, in vapor-compression circuits, such as those which are used in particular in motor vehicle air-conditioning, requires the use of multilayer structures that have a very specific combination of mechanical, thermal and chemical properties.
The elements of these circuits (and in particular the multilayer tubular structures) must in particular:                be impermeable to the fluids transported and therefore have barrier properties with respect to these fluids (and in particular to fluorocarbon refrigerant compounds such as R134, R-1234yf or R-1234ze), and also to water and to oxygen;        have chemical resistance to the fluids transported, and also to the compressor oils, to water and to oxygen, in order to avoid any excessive degradation over the long term;        have sufficient mechanical strength (in particular bursting strength) but also sufficient flexibility in the case where the two ends of the tube are connected to parts that can move relative to one another (in particular in the motor vehicle air-conditioning field, where the constraints of bulkiness and assembly under the hood impose folding the tubular structures) and allow the damping of vibrations;        have satisfactory heat resistance, given the fact that the fluids transported can be at a high temperature, and that the surrounding temperature can also be high (in particular in motor vehicle air-conditioning, the parts concerned possibly being arranged close to the engine).        
At the current time, the tubular structures for transporting heat transfer fluids and in particular refrigerant fluids in motor vehicle air-conditioning comprise rigid metal portions (generally made of aluminum) and flexible portions made of multilayer tubes. Some of these multilayer tubes are known as “veneer” tubes; they successively comprise, from the inside to the outside, an internal barrier layer based on polyamide (or PA), an intermediate layer of elastomer of rubber type, a reinforcing braid and, finally, another layer of elastomer of rubber type.
In the commercially available models, polyamide-based internal layer can for example be a formulated PA 6 (polycaprolactam) (with or without plasticizer, with or without impact modifier, with or without stabilizer), a formulated copolyamide PA 6/66 (with or without plasticizer, with or without impact modifier, with or without stabilizer), or else an alloy of PA 6 or of PA 66 (polyhexamethylene adipamide) with polyolefins and functionalized polyolefins (product sold under the brand name Orgalloy®).
Moreover, document U.S. 2011/0183095 describes a tube or a seal for transporting heat transfer fluids in motor vehicle air-conditioning, comprising an internal layer based on PA 610 (polyhexamethylene sebacamide) and an external layer based on a polyamide such as a polyphthalamide, and preferably the seal is composed of PA 610 reinforced with fibers.
Document U.S. 2011/0272854 describes an article comprising an element composed of fiber-reinforced polyamide overmolded with another element composed of polyamide with optionally a tie between the two elements and where at least one of the polyamides is a semi-aromatic polyamide.
Document EP 1 717 022 relates to multilayer tubes for various applications, and more particularly for transporting fuel in motor vehicles, from the tank to the engine. These tubes comprise an intermediate layer of polyamide, for example of PA 610 (polyhexamethylene sebacamide) or PA 612 (polyhexamethylene dodecanamide).
Document WO 2014/125218 relates to the use of a layer consisting of a composition comprising a copolyamide of formula X/10T/Y for transporting heat transfer fluid in a vapor-compression circuit.
Document WO 2014/125219 relates to a thermoplastic structure comprising at least one layer consisting of a composition based on semi-aromatic copolyamide. The thermoplastic structure is in particular suitable for transporting heat transfer fluid (such as R-1234yf), in particular in the motor vehicle air-conditioning field.
There is a need to develop vapor-compression circuit elements, therefore tubular structures, for transporting transfer fluid, in particular refrigerant fluid such as R-1234yf or R-1234ze, which make it possible to satisfy the specifications in terms of properties of impermeability to the fluids transported (and in particular to refrigerant compounds such as R-1234yf or R-1234ze), and also to water and to oxygen, of chemical resistance to the fluids transported, and also to the compressor oils, water and oxygen, and of mechanical properties, optionally flexibility properties, and heat resistance properties, but especially to simplify and reduce these multilayer structures.
Indeed, the need to reduce weight is particularly felt in the motor vehicle air-conditioning field, more particularly when the heat transfer fluid is R-1234yf or R-1234ze, since it is a question of improving the overall environmental impact of the vehicle, that is to say the impact of the content and therefore of the heat transfer fluid, but more globally of the container, that is to say of the vehicle, and therefore of the structure transporting the heat transfer fluid.
There is moreover a need to simplify the processing of the vapor-compression circuit elements, in particular in motor vehicle air-conditioning.